The present invention relates to spacecraft which are suitable for being maintained in orbit, and especially telecommunication satellites, of the kind designed to fulfill several missions, that is to say of the kind comprising a platform and a multiplicity of different payloads including several telecommunication antennas, comprising at least one feed system and a main reflector.
At present, in telecommunication satellite systems multimission payloads, or a multiplicity of different payloads are often integrated on the same platform.
The reasons for this situation are mainly economic. The economic advantage derives from factors such as : standardisation of platforms, re-using common elements of the platform for the different missions, reduction of operational complexity through the control of a single spacecraft, instead of several ones, reduction of the number of launches.
In the future, more capable launchers, such as further developments of the European Ariane family, and the operational use of the Shuttle of the United States of America, complemented by new Orbit-transfer vehicles, will boost the use of geo-orbit larger platforms. Docking techniques in geostationary-orbit will also permit the building-up of larger platforms with standard launchers, and the growth of systems already in orbit with extensive re-use of common elements of the plateform.
With the advent of larger platforms, the use of multimission systems will be further extended.
In the present, and in the projected systems, there are however some problems of technical and system nature.
Thus, firstly, a multimission spacecraft requires a number of different antennas to satisfy the mission coverage requirements of the different payloads operative at different frequencies.
These antennas present problems of mutual mechanical and electrical interference, which will increase with the use of larger antennas in the future.
For this reasons, in the long-term oriented configurations, often, a number of booms are foreseen to separate the different antennas when their size and number are large.
However, this solution to the interference problem presents the draw backs of:
technological problems in the design of the booms.
increase of weight of the platform with a consequent reduction of the advantage of using common platform.
long feeders from the communication electronic units to the antenna systems or long supply lines when the payloads are integrated in the antenna system.
sophisticated control and stabilisation systems due to decentralisation of time-variable masses in the spacecraft structure, when in-orbit maintenance is foreseen.
The second problem resides in the size of multiple antennas. Reflectors may be so large, in the future, to reduce, in relative terms, the economic advantage of a common platform, if every telecommunication mission will need a separate antenna system.
Thirdly, in case of in-orbit maintenance, the operation of substituting a full payload, including the large antenna system, may be cumbersome and with reduced economic advantage.
This point is allied to a final, but maybe the most important consideration which concerns the life-time of the different components of a space segment. Thus in the present system and in all future foreseen systems, the space segment is divided in two parts: payload and platform.
The antenna system is considered as part of the payload. The requirement, so far, has been to increase the life-time of the global space segment. Life-time has been increased from 3, to 5, to 7, and, in the near future, maybe to 10 years. However, this increase of life-time, through future technology improvements, redundancy policy, in-orbit maintenance, and other sophisticated techniques, has a limit. This limit stems from the telecommunication mission life-time.
While an increase of life-time of the platform is always a positive fact, an increase of life-time of the payload, after a certain limit, is useless, thus economically negative. This is due to the variation of service requirements, the necessity to optimize continuously the use of frequency spectrum and orbit, without increasing the complexity of the ground segment. There are exceptions to this rule, but they are limited to time invariant telecom system such as TVBS systems, at the limit of expansion foreseen by the Geneva 1977 Plan. This is not the case in most applications, particularly in the fixed service area. In this area, there is expected a large growth of variable pattern traffic in the future.
These last considerations imply the preference for systems with platforms designed to have a long life-time, while it should be possible to substitute the payloads with more up-to-date versions, after a limited number of years, through docking techniques.
However, this implies the substitution of a part of the space segment of high cost, weight, and volume, if the whole payload (including the reflector system) is substituted.